Different colours with no texture
The reason for this shading is due to your lighting. The sun lamp you have is casting light rays straight down. If you look closely, the light areas are the areas that receive light, and the dark areas are shadowed.

If you want more even light, try a hemi lamp (I find these to be great for quick, even lighting) or rotate your sun lamp a bit and add a floor plane so the light can reflect and bounce around some more, which will also give some better lighting.